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Minnesota's Impaired Waters and TMDLs

TMDL project: Lake Pepin — Excess nutrients

Lake Pepin is the naturally widest part of the Mississippi River, bordered by Minnesota on the west and Wisconsin on the east. It is located about 60 miles downstream of St. Paul, Minn., just south of the confluence of the St. Croix and Minnesota rivers into the Mississippi. The lake is 21 miles long, averages 1.7 miles wide and covers 29,295 acres. It has a maximum depth of 60 feet and an average depth of 21 feet.

It is a naturally occurring lake formed by the backup of water behind sedimentary deposit of the Chippewa River's delta on the Wisconsin side. It is part of Pool 4 which extends from Lock and Dam 3 near Red Wing, Minn., to Lock and Dam 4 downstream at Alma, Wis.

The villages of Maiden Rock and Stockholm border on the Wisconsin side, while Frontenac State Park takes up a large part of the Minnesota side. The largest city on the waterfront is Lake City, Minn.

About 48,634 square miles – roughly half of Minnesota’s total land area plus a small portion of Wisconsin – including the Upper Mississippi, St. Croix and Minnesota rivers, drain into the Lake Pepin. A number of small streams in southeast Minnesota, many of them coldwater trout streams, drain directly to the lake, making up the Mississippi River - Lake Pepin watershed.

TMDL study. The MPCA assessed the lake in 2004 and found nutrient levels too high to meet state water quality standards designed to ensure that lakes and streams are fishable and swimmable. The assessment led to a Total Maximum Daily Load study to define the maximum amount of a pollutant that the lake can carry without violating water quality standards.

Lake Pepin, because of its massive watershed and complex ecosystem, has been a significant investment of resources for Minnesota. This lake is important to the state for economic, environmental and cultural reasons. While the MPCA continues the TMDL study, state agencies and local partners have implemented many projects to reduce the level of nutrients in rivers that feed Lake Pepin. These projects having a rippling effect all the way to the Gulf of Mexico where high nutrient levels have created a hypoxic area or "dead zone."

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Lake Pepin TMDL project

Lake Pepin is impaired by high levels of nutrients that cause excessive growth of algae. High levels of sediment, carried in by major river systems, also affect the lake. The sediment is filling in the lake at a much faster rate than before Minnesota was settled and intensely farmed. Nutrients and sediment are distinct yet inter-related pollutants, and will be addressed in separate TMDL reports.

Based on research, the site specific standard for Lake Pepin consists of:

100 micrograms per liter for total phosphorus; and

28 micrograms per liter for chlorophyll-a

The MPCA believes these criteria will provide protection of aquatic recreational uses for Lake Pepin and the downstream pools, and should be applicable over the range of flows for which the criteria were developed.

For perspective, the mean level of total phosphorus in Lake Pepin from 2000-2009 was 171 micrograms per liter. Reductions in wastewater phosphorus loading over this same period resulted in Mississippi River concentrations approaching 150 micrograms per liter in 2009. From 2000-2009, the mean of chlorophyll-a was 30 micrograms per liter.

Achieving the site specific standard for Lake Pepin will mean reducing phosphorus and chlorophyll in specific upstream watersheds, such as the Lower Minnesota, Crow and Sauk rivers. To quote the Executive Summary of the criteria document, "The proposed Lake Pepin criteria should not be used in isolation to imply that phosphorus reductions anywhere upstream of the lake will have the desired impact. The main biological activity affecting Lake Pepin trophic status is not taking place in the lake, but upstream of it." In particular, reductions upstream of the Minneapolis-St. Paul wastewater treatment plants will be needed to achieve the desired standards.

The Lake Pepin criteria are not stand-alone goals to be pursued in isolation. Rather, they are part of the goals for the Mississippi River system, which, if pursued systematically in unison, will achieve the desired results.

More specific information about Lake Pepin impairments and this project is available in the following documents: